RPM jumping with deforst on

96whitesvtcobra · February 21st, 2010, 03:53 PM

Ok so I have had a probleam with my car idling up and down for the last few months. I have replace the IAC valve and done a complete tune up and it is still there. The idle jumps about 400 rpms up and down it seemed to only do this with the heater on. Will last night I noticed it only did it when you had it on deforst or mix not on vent or just floor. It even jumps when your not at idle at like going 20 miles an hour though tone you can fill the car idling up. I know the vents are controlled by vaccum and I know on my 96 jimmy the knob selector went bad and caused a vaccum leak. Could this be the same thing? If so what it the part called that controls the ac/heater vents?

ReverendDexter · February 21st, 2010, 03:55 PM

When the A/C is kicked on, the idle is raised to compensate for the extra parasitic drag.

When the defroster is kicked on, it also kicks on the A/C, as air from the air conditioner is dried, and thus defrosts better.

In short, this is normal

96whitesvtcobra · February 21st, 2010, 04:16 PM

It is not the fact that the idle is rasied that is my concern it is that it jumps up and down. It is not stable with the deforst on.

97mustanggt · February 22nd, 2010, 03:15 PM

my idle goes pretty high as well when i turn on my defroster

mustanginky · February 22nd, 2010, 05:33 PM

If it's jumpin up and down you either have low 134 or the low pressure switch is bad. You can jump it to tell. Turn on just the ac I bet it'll do the same thing. The clutch is kicking on and off, raising the rpms with it.

96whitesvtcobra · February 22nd, 2010, 07:31 PM

I just tested this and it does do it when the AC is on also. Just winter so I did not notice that.

. I checked the ac and according to my gauage it is in a acceptable range. How can I test the low pressure switch?

mustanginky · February 22nd, 2010, 07:55 PM

You can jump I somehow, can't remember exactly how. Search it should come up

96whitesvtcobra · February 23rd, 2010, 02:10 PM

Just to get all the info in one place you jump it just by jumping from one wire to the other. There is only two wires on it. This was the issue with mine. Thank you very much. Rep points for you.

mustanginky · February 23rd, 2010, 08:03 PM

if it's in acceptable range then hopefully that's your issue.

my car isn't in the acceptable range, it's gonna take some work. don't know where i got a leak from or how to find it. sucks.

96whitesvtcobra · February 24th, 2010, 01:09 AM

I'm sure you already thought of this but I know here they sale r134a with a dye in it so you can find leaks just a idea. Had to use that on my pickup.

97mustanggt · February 24th, 2010, 01:36 PM

mine has a leak or something as well, it only works when it wants 2

Trick Tuners · February 24th, 2010, 02:08 PM

Maybe its a vacuum leak? Does the rpm jump only when the compressor is cycling? Maybe the compressor is on its way out.

97mustanggt · February 24th, 2010, 02:14 PM

my rpms go up when compressor decides to work

Trick Tuners · February 24th, 2010, 02:15 PM

Originally Posted by 97mustanggt

my rpms go up when compressor decides to work

That sounds pretty normal to me, but 400 rpms is a little high. Are you tuned? If so, you could ask your tuner to lower it a bit.

February 21st, 2010, 03:53 PM	#1
96whitesvtcobra Regular 1996 Mustang Cobra Join Date: Mar 2009 Location: El dorado springs mo Posts: 151 iTrader: 0 reviews	RPM jumping with deforst on Ok so I have had a probleam with my car idling up and down for the last few months. I have replace the IAC valve and done a complete tune up and it is still there. The idle jumps about 400 rpms up and down it seemed to only do this with the heater on. Will last night I noticed it only did it when you had it on deforst or mix not on vent or just floor. It even jumps when your not at idle at like going 20 miles an hour though tone you can fill the car idling up. I know the vents are controlled by vaccum and I know on my 96 jimmy the knob selector went bad and caused a vaccum leak. Could this be the same thing? If so what it the part called that controls the ac/heater vents?

February 21st, 2010, 03:55 PM	#2
ReverendDexter Priest of the Car Gods Clapped-out '90 GT 'vert ~5.5 seconds off TTOD :( Join Date: Jan 2007 Location: Rohnert Park, CA Posts: 13,808 iTrader: 0 reviews	When the A/C is kicked on, the idle is raised to compensate for the extra parasitic drag. When the defroster is kicked on, it also kicks on the A/C, as air from the air conditioner is dried, and thus defrosts better. In short, this is normal __________________ O o /¯/______________________ \|BLAAAAAAAAAAAAAAAAAAAAAAAAARGH!!! \_\

February 22nd, 2010, 03:15 PM	#4
97mustanggt Hardcore Enthusiast 97GT & 97Snake & 00L T.B.A Join Date: Aug 2008 Location: Richgrove, Ca Posts: 2,183 iTrader: 0 reviews	my idle goes pretty high as well when i turn on my defroster __________________ 97 Cobra & 00 Lightning Beauty & The Beast I rev next to hybrids so they can hear me kill the environment MM Cobra Club Member #77 MM Lightning Club Founder

February 22nd, 2010, 05:33 PM	#5
mustanginky MM Fanatic 2000 gt Join Date: Feb 2009 Location: La grange, KY Posts: 4,254 iTrader: 2 reviews	If it's jumpin up and down you either have low 134 or the low pressure switch is bad. You can jump it to tell. Turn on just the ac I bet it'll do the same thing. The clutch is kicking on and off, raising the rpms with it. __________________ Procharged 2000 5 Speed BAB GT, built motor, blower cams, speedy dyno tune 412 hp, 450 rw tq HARDCORE CREW BRIGHT ATLANTIC BLUE CREW #1 MEMBER

February 22nd, 2010, 07:55 PM	#7
mustanginky MM Fanatic 2000 gt Join Date: Feb 2009 Location: La grange, KY Posts: 4,254 iTrader: 2 reviews	You can jump I somehow, can't remember exactly how. Search it should come up __________________ Procharged 2000 5 Speed BAB GT, built motor, blower cams, speedy dyno tune 412 hp, 450 rw tq HARDCORE CREW BRIGHT ATLANTIC BLUE CREW #1 MEMBER

February 21st, 2010, 04:16 PM	#3
96whitesvtcobra Regular 1996 Mustang Cobra Join Date: Mar 2009 Location: El dorado springs mo Posts: 151 iTrader: 0 reviews	It is not the fact that the idle is rasied that is my concern it is that it jumps up and down. It is not stable with the deforst on.

February 22nd, 2010, 07:31 PM	#6
96whitesvtcobra Regular 1996 Mustang Cobra Join Date: Mar 2009 Location: El dorado springs mo Posts: 151 iTrader: 0 reviews	I just tested this and it does do it when the AC is on also. Just winter so I did not notice that. . I checked the ac and according to my gauage it is in a acceptable range. How can I test the low pressure switch?

February 23rd, 2010, 02:10 PM	#8
96whitesvtcobra Regular 1996 Mustang Cobra Join Date: Mar 2009 Location: El dorado springs mo Posts: 151 iTrader: 0 reviews	Just to get all the info in one place you jump it just by jumping from one wire to the other. There is only two wires on it. This was the issue with mine. Thank you very much. Rep points for you.

February 23rd, 2010, 08:03 PM	#9
mustanginky MM Fanatic 2000 gt Join Date: Feb 2009 Location: La grange, KY Posts: 4,254 iTrader: 2 reviews	if it's in acceptable range then hopefully that's your issue. my car isn't in the acceptable range, it's gonna take some work. don't know where i got a leak from or how to find it. sucks. __________________ Procharged 2000 5 Speed BAB GT, built motor, blower cams, speedy dyno tune 412 hp, 450 rw tq HARDCORE CREW BRIGHT ATLANTIC BLUE CREW #1 MEMBER

February 24th, 2010, 01:09 AM	#10
96whitesvtcobra Regular 1996 Mustang Cobra Join Date: Mar 2009 Location: El dorado springs mo Posts: 151 iTrader: 0 reviews	I'm sure you already thought of this but I know here they sale r134a with a dye in it so you can find leaks just a idea. Had to use that on my pickup.

February 24th, 2010, 01:36 PM	#11
97mustanggt Hardcore Enthusiast 97GT & 97Snake & 00L T.B.A Join Date: Aug 2008 Location: Richgrove, Ca Posts: 2,183 iTrader: 0 reviews	mine has a leak or something as well, it only works when it wants 2 __________________ 97 Cobra & 00 Lightning Beauty & The Beast I rev next to hybrids so they can hear me kill the environment MM Cobra Club Member #77 MM Lightning Club Founder

February 24th, 2010, 02:08 PM	#12
Trick Tuners MM Fanatic KB'd GT Join Date: Nov 2006 Location: San Diego Posts: 4,467 iTrader: 3 reviews	Maybe its a vacuum leak? Does the rpm jump only when the compressor is cycling? Maybe the compressor is on its way out. __________________ Check out my forums for stuff on troubleshooting, tuning, etc We teamed up with Socal Performance Racing to bring dyno tuning to San Diego. Be friends with us on FB

February 24th, 2010, 02:14 PM	#13
97mustanggt Hardcore Enthusiast 97GT & 97Snake & 00L T.B.A Join Date: Aug 2008 Location: Richgrove, Ca Posts: 2,183 iTrader: 0 reviews	my rpms go up when compressor decides to work __________________ 97 Cobra & 00 Lightning Beauty & The Beast I rev next to hybrids so they can hear me kill the environment MM Cobra Club Member #77 MM Lightning Club Founder

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